The Corey–Pauling rules
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
675
2025-12-18
48
The Corey–Pauling rules
The origin of the secondary structures of proteins is found in the rules formulated by Linus Pauling and Robert Corey in 1951. The essential feature is the stabilization of structures by hydrogen bonds involving the peptide link. The latter can act both as a donor of the H atom (the NH part of the link) and as an acceptor (the CO part). The Corey–Pauling rules are as follows (Fig. 19.25):
1 The four atoms of the peptide link lie in a relatively rigid plane. The planarity of the link is due to delocalization of π electrons over the O, C, and N atoms and the maintenance of maximum overlap of their p orbitals.
2 The N, H, and O atoms of a hydrogen bond lie in a straight line (with displace ments of H tolerated up to not more than 30° from the N-O vector).
3 All NH and CO groups are engaged in hydrogen bonding. The rules are satisfied by two structures. One, in which hydrogen bonding between peptide links leads to a helical structure, is a helix, which can be arranged as either a right- or a left-handed screw. The other, in which hydrogen bonding between peptide links leads to a planar structure, is a sheet; this form is the secondary structure of the protein fibroin, the constituent of silk.
Fig. 19.25 The dimensions that characterize the peptide link (bonds in picometres). The C-NH-CO-C atoms define a plane (the C-N bond has partial double-bond character), but there is rotational freedom around the C-CO and N–C bonds.
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